Subzero: Add a script that builds a hybrid Subzero/llc native executable.

The script translates a pexe using both Subzero and llc, and then uses linker tricks to resolve each symbol into either its Subzero or llc version.  This enables quick bisection-based debugging of Subzero code generation.

BUG= none
R=jfb@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/551953002